Homeland Security Secretary Janet Napolitano testifies before the House Homeland Security Committee hearing on the "President's FY2012 Budget Request for the Department of Homeland Security" at the Canon House Office building in Washington, DC, on March 3, 2011.

Photo: JEWEL SAMAD, AFP/Getty Images

Homeland Security Secretary Janet Napolitano testifies before the...

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Michael McCaul in 2008, in Houston.

Photo: Steve Ueckert, Chronicle

Michael McCaul in 2008, in Houston.

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ICE Special Agent Jaime Zapata was shot and killed February 15 after he was attacked by unknown assailants while driving between Monterrey, Mexico, and Mexico City.

Photo: AFP/Getty Images

ICE Special Agent Jaime Zapata was shot and killed February 15...

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Bishop of the Diocese of Brownsville, Daniel E. Flores conducts the funeral Mass for Immigration and Customs Enforcement Special Agent Jaime Zapata at the Brownsville Special Events Center in Brownsville Texas Tuesday, Feb. 22, 2011.

Two women bow before the casket of slain Immigration and Customs Enforcement Special Agent Jaime Zapata during a funeral Mass at the Brownsville Special Events Center in Brownsville Texas Tuesday, Feb. 22, 2011.

Photo: BRAD DOHERTY, AP

Two women bow before the casket of slain Immigration and Customs...

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An honor guardsman stands next to a U.S. Department of Homeland Security flag and the casket of slain Immigration and Customs Enforcement Special Agent Jaime Zapata before his funeral Mass at the Brownsville Special Events Center Tuesday, Feb. 22, 2011 in Brownsville, Texas.

Photo: BRAD DOHERTY, AP

An honor guardsman stands next to a U.S. Department of Homeland...

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Mexican federal police and army soldiers guard a U.S. Embassy vehicle after it came under attack by unknown gunmen on Highway 57 between Mexico City and Monterrey, near the town of Santa Maria Del Rio, San Luis Potosi state, Mexico, Tuesday Feb. 15, 2011.

MEXICO CITY — In his final moments, colleagues said, U.S. federal agent Jaime Zapata struggled to fight off his Mexican killers as they attempted to yank him from the armored embassy Suburban he was driving.

Zapata and Victor Avila, a fellow Immigration and Customs Enforcement agent, had been forced off the road Tuesday afternoon by two carloads of gunmen as they traveled the busy toll expressway connecting Mexico's heartland to the South Texas border.

They hoped to reason with the gunmen - as many as 15 of them in fatigues - who surrounded the vehicle, U.S. officials said.

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"The agent said, you know, 'We're Americans, we're diplomats,' and the response from the drug cartels was bullets," said U.S. Rep. Michael McCaul, R-Austin, who heads a subcommittee of the House Committee on Homeland Security and was briefed Wednesday on the incident by senior officials in Washington.

"They put an AK-47, as I understand it, into the crack in the window and just began to fire away," McCaul recounted.

Details of the assault also were provided to the Houston Chronicle by current and former federal agents who reviewed official reports. The attack on U.S. law enforcement officers in the line of duty, in north central San Luis Potosi state some 500 miles south of the border at Laredo, is the first of its kind in Mexico in 26 years.

Avila took two bullets to his leg in the attack. Flown to Houston for treatment, he was released from Ben Taub hospital Wednesday.

'He was wonderful'

A native of Brownsville, Zapata had joined ICE in 2006 and usually worked out of the agency's Laredo office. He's survived by his family in Brownsville.

The agents were driving a sport-utility vehicle with diplomatic plates, leading to strong speculation they were targeted because they were U.S. law enforcement officers, though such a move by the gunmen would be bold and reckless, provoking the full wrath of the U.S. government.

"I think all hell is going to break loose," predicted one federal agent not authorized to talk on the record.

Mexican authorities are leading the investigation.

According to agents' and officials' accounts in the U.S., Zapata and Avila had stopped for lunch and were just back on the Pan American Highway when the gunmen's cars pulled alongside them before speeding ahead. A few miles farther on, the gunmen lay in ambush, their cars bracketing the agents' vehicle in front and behind, forcing it onto the grassy median.

When Zapata shifted the vehicle into park, its doors automatically unlocked. Gangsters pulled open the driver's side door and tried to drag Zapata out.

He fought them off, managing to re-lock the doors. The agents then apparently cracked the front passenger window to talk with the assailants and identify themselves. The gangsters fired through the opening, hitting both agents.

U.S. response

Dying, Zapata put the car in gear and sped off by pushing the assailants' SUV out of the way before collapsing at the wheel. The vehicle coasted to a stop in the median. The agents had used a mobile phone to call for help, and a Mexican federal police helicopter arrived soon afterward.

The gunmen fled.

"They told the bad guys, 'Diplomats, diplomats,' " said one former federal agent who also reviewed the file on the incident. "Apparently the bad guys, in Spanish, told them they don't give a (expletive)."

No one doubts the U.S. response to Zapata's death will be fierce. It also has the potential to drive a wedge into the delicate alliance that has been forged by both governments to defeat Mexico's powerful criminal syndicates, many say.

"Clearly the U.S. government gives a great deal of importance to the protection of its agents," said Jorge Chabat, a national security expert in Mexico City. "In this sense it's going to provoke a crisis in the relationship."

Zapata's killers have yet to be identified. But many believe they are a local cell of the Zetas, the assassins-turned-drug-traffickers based in the cities bordering South Texas whose sway extends across much of Mexico.

The Zetas recently have been fighting rivals and federal security forces in once-tranquil San Luis Potosi state. Gangland battles killed at least 120 people last year in the area. Dozens more have died since early January, in clashes that include two with Mexican soldiers on the very highway where the agents were assaulted.

'Hunted down at all costs'

Whether aimed at U.S. officials or not, the gunmen's attack may be something they'll regret, former U.S. officials say.

"Any drug trafficker with any sense of intelligence would not commit such a heinous crime as this, knowing full well that there is going to be consequences," said Mike Vigil, the retired international operations director for the Drug Enforcement Administration.

"They will be hunted down at all costs," said Vigil, who spent years working against Mexico's criminal cartels. "All of the law enforcement agencies come together in a common effort. They will be brought to justice, no doubt about it."